Hitherto, for the purpose of detecting the position of a cylinder rod of a construction or industrial machine, a position detecting apparatus has been often used in which a variation in the magnetic characteristics is created at a predetermined spacing along the cylinder rod or a detecting rod connected thereto, by attaching magnets to the rod or varying the magnetic permeability at the predetermined spacing, whereby the displacement or position of the rod is detected on the basis of the change in the magnetic characteristics. The change in the magnetic characteristics can be detected by various detecting means such as, for example, a detecting means using a Maxwell bridge circuit. The Maxwell bridge circuit has four sides, one of which is formed by a coil having a reference inductance L.sub.1, and a coil having an unknown value of inductance L.sub.3 is inserted in another side of the bridge. In operation, an adjustment is conducted to nullify the output voltage between the output terminals, thereby determining the inductance L.sub.3.
More specifically, a known position detecting apparatus incorporating a Maxwell bridge circuit has a detecting rod made of a magnetically permeable metal. The detecting rod has a plurality of magnetic permeability changing portions which are formed by locally increasing or decreasing magnetic permeability at a predetermined pitch along the rod. The detecting rod is received in the coil having the unknown inductance L.sub.3. When the detecting rod is axially moved to cause the magnetic permeability changing portion to pass through the coil, the self-inductance L.sub.3 of the coil is changed to cause a difference between the voltage across this coil and the voltage across the coil having the inductance L.sub.1. Consequently, an A.C. voltage produced by this phase difference is obtained from the output terminals, and the amount of displacement, i.e., the position, of the detecting rod is determined on the basis of the zero-cross point of the A.C. voltage.
The voltage between the output terminals of the Maxwell bridge circuit should be zero when there is no displacement of the detecting rod. Actually, however, a voltage appears between the output terminals even when there is no displacement of the detecting rod, due to variations or fluctuations of various factors such as the sensor sensitivity, the state of mounting of the detecting rod, the magnetic permeability of the detecting rod, and the characteristics of the circuit elements, as well as the temperature at which the detecting apparatus operates, with the result that the zero-cross point is deviated. Thus, the output voltage obtained from the Maxwell bridge circuit in response to an axial movement of the detecting rod is the sum of the A.C. voltage generated as a result of passage of the magnetic permeability changing portion through the coil and a D.C. voltage (offset voltage) which is attributable to the offset of the zero-cross point caused by fluctuations in the factors such as the sensor sensitivity and the characteristics of the circuit components. Therefore, the determination of the position or displacement of the detecting rod through the detection of the zero-cross point of the A.C. output voltage from a Maxwell bridge circuit essentially requires that the offset voltage component be removed from the output voltage of the Maxwell bridge circuit.
There are two methods for removing the offset voltage: a method called A.C. coupling method which utilizes a capacitor, and a method called voltage adjusting method which uses a trimmer resistor. The A.C. coupling method, which relies upon charging and discharging of a capacitor, suffers from various disadvantages such as the generation of a delay of a phase or delay of zero-cross of the A.C. voltage due to inferior follow-up characteristics. Furthermore, it is impossible to pickup an A.C. component from the output of the Maxwell bridge circuit, when the frequency of the A.C. component is as low as several Hz or less. On the other hand, the voltage adjusting method which employs a trimmer resistor essentially requires a complicated voltage adjustment, particularly when the frequency of the output voltage of the Maxwell bridge circuit is changed or when the center of the amplitude of the output is varied due to a variation in the offset voltage. This problem is serious, particularly when a cylinder rod is used directly as the detecting rod, since in such a case the offset voltage is varied as a result of a change in the temperature of the detecting rod, with the result that the adjustment of the voltage is further complicated.
In view of these drawbacks of the known art, the present inventor has proposed a method in which, as shown in FIG. 6, a mean value of the output voltage of the detecting circuit including a Maxwell bridge circuit is used as a reference voltage of a comparator circuit.
Referring to FIG. 6, the detecting circuit 10 is composed of a Maxwell bridge circuit 12, a phase detector 14 and a differential amplifier 16, and is connected to the output of an oscillator 18 which serves as an A.C. power supply. The oscillator 18 produces a high-frequency voltage of, for example, 60 KHz, and applies this voltage to the Maxwell bridge circuit 12 and also to the phase detector 14. A voltage appearing between the output terminals a and b of the Maxwell bridge circuit 12 is amplified by a differential amplifier 16, the output of which is delivered to the phase detector 14.
The phase detector 14 determines the phase difference between the output voltage of the oscillator 18 and the output voltage of the differential amplifier 16 and produces, as its output signal, a voltage corresponding to the phase difference. The output voltage of the phase detector 14 is delivered to a noise filter 20 for removing the noise of the output voltage and also to a mean value calculating circuit 22.
The mean value calculating circuit 22 is composed of a low-pass active filter of voltage controlled voltage source type (referred to as VCVS, hereinafter) having Butterworth characteristic, and is composed of an RC passive circuit 24 and an operational amplifier 26. The output of the operational amplifier 26 is delivered to a voltage holding circuit 28. The voltage holding circuit 28 receives a one-shot pulse from a voltage holding timing generating circuit 30 which produces voltage holding timing on the basis of the output from the noise filter 20. Upon receipt of this one-shot pulse, the voltage holding circuit 28 stores the mean voltage which is outputted from the mean value calculating circuit 22 and holds this value until the next one-shot pulse is received. The voltage holding circuit 28 also delivers this mean voltage as a reference voltage to a comparator circuit 32 which also receives the output from the noise filter 20.
Using the mean value of the output from the detecting circuit 10 as the reference voltage of the comparator circuit 32, it is possible to automatically adjust the offset voltage appearing between the output terminals a and b of the Maxwell bridge 12.
The low-pass active filter of VCVS type which forms the mean value calculating circuit 22 has a time constant R.sub.1 R.sub.2 C.sub.1 C.sub.2 and the frequency f.sub.0 is expressed by: ##EQU1##
This frequency f.sub.0 is the center frequency of the filtering. The frequency of the output voltage of the detecting circuit 10 is varied by the velocity of the movement of the detecting rod which is not shown and which is received in the coil L.sub.3 of the Maxwell bridge circuit 12. Therefore, when the mean value of the output voltage of the detecting circuit 10 is calculated by the mean value calculating circuit 22, it is necessary to change the center frequency f.sub.0 of the low-pass active filter in accordance with the velocity of the movement of the detecting rod. It is quite difficult to vary the center frequency f.sub.0 in accordance with the velocity of movement of the detecting rod. It is therefore desirable that the low-pass active filter be capable of smoothing all the frequencies which are outputted from the detecting circuit 10 in accordance with any change in the velocity of the movement of the detecting rod.
On the other hand, the velocity of the movement of the detecting rod is low when the movement is started, as in the case of a cylinder rod. The use of the low-pass active filter is therefore necessary for determining the mean value of the output voltage of the detecting circuit 10. Actually, however, the velocity of the detecting rod varies over a wide range and so does the frequency of the output voltage of the detecting circuit 10. Therefore, when the mean value of the output of the detecting circuit in the high-speed range is calculated with the aid of the low-pass filter, a delay of smoothing calculation (delay of response) is caused due to time constant, although the calculation for determining the mean value is carried out.
FIG. 7 illustrates an example of such a delay in the response. It will be seen that 300 m sec is required until the mean value calculating circuit 22 outputs the mean value of the output voltage of the detecting circuit 10 after the moment at which the detecting rod which has been kept stationary starts to move.
The present invention is aimed at overcoming the above-described problems of the prior art. An object of the present invention is to provide a position detecting apparatus which is capable of reducing delay of response of the low-pass active filter.